1. Field of the Invention
The present invention relates to manufacturing method of a single-crystal silicon carbide (SiC), especially to a method capable of restraining a micro-pipe as a crystal defect from being inherited.
2. Related Art
Heretofore, SiC single crystals are generally produced by sublimation method, however, hollow penetrating holes called micro-pipe defects (hollow penetrating defects) are formed at a degree of 100 to 1000 pieces/cm2.
In a case where a power device or a high frequency device is formed, an epitaxial film, which becomes regions to form devices, is formed so as to have a structure suitable for the devices using these single crystals as a substrate. When the defects exist in the substrate, the defects are inherited into the epitaxial film which is grown on the substrate, so that defects which are in the same number of the micro-pipe defects are formed in the epitaxial film. Further, when the devices are formed in this epitaxial film with these defects, the fact is reported that leak current of the devices increases while backward withstand voltage decreases. Therefore, it is very important to reduce the defects in producing the devices.
As a method for reducing the micro-pipe defects in the epitaxial film in which devices are formed, recently, methods for eliminating the micro-pipe defects in the SiC single crystals as the substrate has been proposed. The methods are disclosed in U.S. Pat. No. 5,679,153, JP-A-10-324600, JP-A-2000-44398, and xe2x80x9cStudy on dislocations of 4H-SiC thick layer grown by CVD xe2x80x9d (The Lecture of the 47th Japan Society of Applied Physics Related Association, Abstracts of the lecture, separate volume 1, page 407, No. 29P-YF-6, Kamata et al., March, 2000, Central Research Institute of Electric Power Industry).
According to the method in U.S. Pat. No. 5,679,153, when crystals are grown by liquid crystal epitaxy technique using melted SiC in silicon, an epitaxial film in which micro-pipe defects are reduced is grown on a seed substrate having micro-pipes.
Next, according to the method in JP-A-10-324600, formation of a polycrystalline film of a xcex2(cubic)-SiC or xcex1(hexagonal)-SiC on a surface of an xcex1-SiC single crystal substrate (seed crystal) by thermal chemical vapor deposition (CVD) and thermal treatment of the composite body resulting from the formation are repeated a plurality of times so that a plurality of xcex1-SiC or xcex2-SiC polycrystalline films are oriented (the kind of solid phase epitaxial growth) in the same direction of the crystal axis of the xcex1-SiC single crystal substrate (seed crystal). Thus, SiC single crystals are formed so as to have few micro-pipe defects.
On the other hand, according to JP-A-2000-44398, after a coating material is coated on a single crystal substrate having micro-pipes, thermal treatment is conducted to occlude the micro-pipe defects in the SiC substrate that exist in the SiC substrate, so that a crystal in which at least a part of the micro-pipe defects are occluded is obtained.
Further, according to the Abstracts in The Lecture of the 47th Japan Society of Applied Physics Related Association, the fact is reported that an epitaxial film is formed on a substrate in a thickness of 65 xcexcm at a rate of 16 xcexcm/h, so that micro-pipes are occluded.
According to the above-described first method, the epitaxial film should be grown to a thickness of about 20 to 75 xcexcm or more by the liquid phase epitaxy method, to obtain a region where the micro-pipes are eliminated. Moreover, an epitaxial film on which devices are formed is formed on the epitaxial film by liquid phase epitaxy by a CVD method, so that a number of manufacturing processes increase.
According to the above-mentioned second method, SiC composite is obtained so as to include crystal boundaries therein since the polycrystalline film is formed on the single crystal substrate. When the composite is subjected to the thermal treatment to cause the solid phase epitaxy on the seed crystal, there is possibility that crystal defects due to internal stress at the crystal boundaries in the polycrystalline film are introduced. These defects become sources of traps, and therefore there is a problem that this substrate is not suitable for a substrate to form devices. Moreover, the formation of the film, the thermal treatment, and a surface flattening should be repeated several times to grow a substrate having a practical thickness. Thus, processes increase so that manufacturing cost becomes high.
According to the above-mentioned third method, at least the covering process with the coating material, the thermal treatment, and a surface flattening process that includes a removing process of the coating material are necessary, so that the manufacturing process increases.
According to the above-mentioned fourth method, although the micro-pipes are occluded by thickening the epitaxial film, a thickness of the epitaxial film to form devices on the substrate is about 20 to 30 xcexcm at most. Therefore, there is a need that the micro-pipes are occluded even if the epitaxial film is thin. Besides, the growth rate only about 16 xcexcm/h. It takes many hours, i.e., 4 hours or more to occlude the micro-pipes. That is, this method is not suitable for a commercial use as a method for forming an epitaxial film for devices or bulk.
The present invention has been made in view of the above-mentioned problem, and an object thereof is to provide a technique for providing more practical occluding method of a micro-pipe in a silicon carbide substrate that has the micro-pipe.
According to a first aspect of the present invention, when an epitaxial film is formed on a silicon carbide substrate having a micro-pipe, temperature difference is applied between a front surface of the substrate and a back surface of the substrate that is disposed opposite to the front surface so that the front surface is at a low temperature.
By lowering the temperature at the front surface of the silicon carbide substrate as compared to the back surface, sublimation gas of SiC is generated at the vicinity of the back surface where the temperature is high (the micro-pipe or the back surface). The sublimated gas flows to a side of the front surface through the micro-pipe, and then recrystallized at the vicinity of the front surface where the temperature is low. At that time, the gas is recrystallized at an inside of the micro-pipe, so that an inner diameter of the micro-pipe becomes small, and finally, the micro-pipe can be occluded.
Incidentally, as described in a second aspect of the present invention, by setting a temperature of the substrate at 1650xc2x0 C. or more, sublimation is apt to occur from the substrate, and occlusion of the micro-pipe is stimulated.
Moreover, as described in a third aspect of the present invention, by setting the temperature of the substrate at 1750xc2x0 C. or more, sublimation is stimulated from the substrate, so that the micro-pipe is occluded easily. However, in a case where the temperature exceeds 1900xc2x0 C., the sublimation is stimulated so that the sublimation and a recrystallization are balanced so as to restrain the occlusion of the micro-pipe. Therefore, the temperature of the silicon carbide substrate is preferably set to 1900xc2x0 C. at most.
Moreover, as described in a fourth aspect of the present invention, since hydrogen gas or helium gas has a high heat-transmitting characteristic so as to effectively lower the temperature at the front surface of the silicon carbide substrate where the gas is supplied, so that the temperature difference between the front surface and the back surface of the substrate is sufficiently generated. Thus, the sublimation gas from the vicinity of the back surface of the silicon carbide substrate is transferred to the front surface.
Moreover, as described in a fifth aspect of the present invention, by setting a flow rate of the gas at 1 m/sec or more, the temperature at the front surface of the silicon carbide substrate where the gas is supplied is effectively lowered so that the temperature difference between the front surface and the back surface of the substrate is sufficiently generated. Thus, the sublimation gas from the vicinity of the back surface of the silicon carbide substrate is transferred to the front surface.
Moreover, as described in a sixth or seventh aspect of the present invention, by setting the temperature difference between the front surface and the back surface of the silicon carbide substrate at 0.5, preferably, 5xc2x0 C. or more, it can be encouraged that the gas sublimed at a side of the back surface is transferred, and recrystallized at the vicinity of the front surface. Thus, the micro-pipe can be occluded easily.
Moreover, as described in a eighth aspect of the present invention, by setting a growth rate of the epitaxial film at 20 xcexcm/h or more, a growth rate toward a lateral direction of a silicon carbide film on the micro-pipe can be enhanced, so that the occlusion of the micro-pipe can be shortened.
Moreover, as described in a ninth aspect of the present invention, when a thickness of the silicon carbide substrate is at 300 xcexcm or more, the temperature at the front surface of the substrate where the gas is supplied is effectively lowered, whereby the temperature difference between the front surface and the back surface of the substrate is sufficiently generated. Thus, it is encouraged that the sublimation gas from the back surface is transferred to the front surface.
Moreover, as described in a tenth aspect of the present invention, by setting flow direction of a gas containing carbon and a gas containing silicon in approximately perpendicular to a front surface of the substrate that exposes an opening of said micro-pipe, a gas sublimed through the micro-pipe from the back surface is prevented from flowing out to the front surface. Therefore, it is encouraged that the sublimed gas is recrystallized at the vicinity of the opening. Thus, the micro-pipe can be occluded easily.
Moreover, as described in an eleventh aspect of the present invention, by setting a temperature of the substrate at 1650xc2x0 C. or more, sublimation is apt to occur from the substrate, so that the micro-pipe can be easily occluded.
Moreover, as described in a twelfth aspect of the present invention, by preferably setting the temperature of the substrate at 1750 to 1900xc2x0 C., sublimation is encouraged from the substrate, so that the micro-pipe is occluded easily. The reason why the temperature is set at 1900xc2x0 C. or less is that the sublimation is more encouraged as compared to the recrystallization at over 1900xc2x0 C., and therefore heating up over 1900xc2x0 C. is not preferable.
Furthermore, as described in a thirteenth aspect of the present invention, the micro-pipe penetrates the substrate from the front surface to the back surface, and the substrate is held so as to closely contact a contacting member at the back surface thereof, so that the sublimed gas from the micro-pipe at the vicinity of the back surface is apt to move to the front surface, whereby the sublimed gas is encouraged to be recrystallized at the vicinity of the front surface. Thus, the micro-pipe can be easily occluded.
Moreover, as described in a fourteenth aspect of the present invention, the substrate is held so that pressure of an atmosphere contacting the back surface is high as compared to that of an atmosphere contacting the front surface. As a result, the sublimed gas from the micro-pipe at the vicinity of the back surface is apt to move to the front surface, whereby the sublimed gas is encouraged to be recrystallized at the vicinity of the front surface. Thus, the micro-pipe can be easily occluded.
Moreover, as described in a fifteenth aspect of the present invention, by reducing the pressure in the epitaxial growth, the pressure at the vicinity of the back surface of the substrate is lowered through the micro-pipe, so that the sublimation of silicon carbide is encouraged. Thus, the micro-pipe can be easily occluded.
Moreover, as described in a sixteenth aspect of the present invention, when an opening of the micro-pipe is enlarged in the silicon carbide substrate, a plurality of steps are formed at the opening. Since the steps are cores, a lateral growth of a silicon carbide film progresses, and therefore the micro-pipe can be easily occluded.
Moreover, as described in a seventeenth aspect of the present invention, by heating up the SiC substrate to 1650xc2x0 C. or more in hydrogen, the front surface of the silicon carbide substrate is etched. Specifically, an etching in the vicinity of defects is encouraged, so that the opening of the micro-pipe can be enlarged. Successively, by supplying a gas containing carbon and a gas containing silicon, the epitaxial film can be grown by epitaxial growth.
Moreover, as described in an eighteenth aspect of the present invention, by supplying a gas containing chlorine, the front surface of the silicon carbide substrate is etched. Specifically, an etching in the vicinity of defects is encouraged, so that the opening of the micro-pipe can be enlarged. Successively, by supplying a gas containing carbon and a gas containing silicon, the epitaxial film can be grown by epitaxial growth.
Moreover, as described in a nineteenth aspect of the present invention, by etching the silicon carbide substrate using KOH, the etching in the vicinity of the defects is encouraged, so that the opening of the micro-pipe can be enlarged.
Moreover, as described in a twentieth aspect of the present invention, when an enlarged diameter in the opening of the micro-pipe has a size of twice or more as large as that before enlarged, the gas can be supplied sufficiently, so that the growth at the opening is encouraged. The micro-pipe can be easily occluded since a plurality of steps can be formed at the opening, and a growth of a silicon carbide film in a lateral direction progresses while the steps serve as cores.
Incidentally, as described in a twenty-first aspect of the present invention, the silicon carbide substrate, on which the silicon carbide film is formed by the epitaxial growth in a chamber, serves as a seed crystal, and a sublimation gas sublimed from a source material is generated in the chamber. Then, the sublimation gas is sublimed on the seed crystal.
Moreover, as described in a twenty-second aspect of the present invention, by forming the epitaxial film on a silicon carbide substrate having a micro-pipe with an opening whose diameter increases as being close to a front surface of said substrate, a substrate can be obtained, in which the silicon carbide epitaxial film is not opened on the micro-pipe. Therefore a high quality silicon carbide single crystal substrate which has less micro-pipe is produced. Besides, the micro-pipe is occluded (or terminated) in this silicon carbide substrate, which can be an advantage for producing devices since a thickness of the epitaxial film is usually considered in producing the devices without considering a location of the micro-pipe.
Incidentally, as described in a twenty-third aspect of the present invention, a diameter of the opening, which is enlarged at a surface of an opening of the substrate, is preferably twice or more as large as that of the opening at a bottom of the opening of the substrate.
Moreover, as described in a twenty-fourth aspect of the present invention, when the micro pipe is occluded (or terminated) at a conductive region disposed between the silicon carbide substrate body and the epitaxial film, and the devices are formed in this substrate, in a case where a voltage is applied so as to expand a depletion layer, the depletion layer that is expanded from the epitaxial layer is restrained from being expanded by the conductive region so that the depletion layer is prevented from reaching the micro-pipe. Therefore, electric field concentration at the micro-pipe, which is caused by the phenomenon in which the depletion layer reaches the micro-pipe, is suppressed so that a breakdown due to the micro-pipe can be prevented.
Incidentally, the conductive region is regarded as a region where an impurity concentration is high in comparison with a predetermined epitaxial film.
Moreover, as described in a twenty-fifth aspect of the present invention, the conductive region may be a conductive substrate, or as described in a twenty-sixth aspect of the present invention, the conductive region may exist in the epitaxial film.
Otherwise, as described in a twenty-seventh aspect of the present invention, the conductive region is a low resistivity epitaxial film. In this case, the depletion layer expanding from a high resistivity epitaxial film formed on the low resistivity epitaxial film is restrained from expanding by the low resistivity epitaxial film. As a result, the breakdown due to the micro-pipe is prevented.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings.